Adult supervision is recommended for making the gelatin, which involves using boiling water and a knife to cut the fruits.

Abstract

Why are some fruits, like pineapple, not recommended for adding to gelatin? It is because the gelatin may not solidify well if it has these fruits in it. In this science project you will determine whether certain enzymes in some fruits are preventing gelatin from solidifying, and whether there is a way to still include these fruits without ruining your gelatin dessert. It is an experiment with edible results!

Objective

Determine if the enzymes in some fruits prevent gelatin from solidifying and if heating these fruits can stop the activity of their enzymes.

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Introduction

If you like making gelatin for dessert, you might have noticed that the box recommends against adding certain kinds of fruit, such as papaya and pineapple, which are shown in Figure 1 below, as well as other fruits, like kiwi, mango, ginger root, figs, or guava. But why? What happens when you add these fruits to your gelatin? Why can you use some fruits, but not others? You will find out for yourself in this science fair project, but if you do a bit of background reading first, you are likely to find that people have a hard time getting the gelatin to solidify when they add certain fruits. To discover why, you first need to know a little about what gelatin is and how it normally sets.

Figure 1.
When making a gelatin dessert, the packaging may recommend against using certain fruits, like papaya (shown on the left here) and pineapple (shown cut up on the right). This is because including these fruits can make it difficult for the gelatin to solidify.

Gelatin is made from a protein called collagen. Proteins are a basic type of matter that make up all living things. Your skin, your blood, your hair— all of these are made up of many different types of proteins. One of those proteins is collagen. Collagen is a structural protein found in all animals, meaning that it helps give animals their structure, or shape. Collagen can be found in many parts of your body, including your skin, bones, muscles, and cartilage. Gelatin is a mixture of collagen proteins that have undergone a chemical reaction that makes them able to solidify when you are cooking with them. When you make a gelatin dessert, you dissolve the gelatin mix in hot water. The collagen proteins in the gelatin are like microscopic (meaning too small to see with just your eyes) strands of spaghetti. These long, thin, flexible proteins tangle up with one another, the same way strands of cooked spaghetti do when they are all in one pot together. As the gelatin proteins tangle, they form mesh pockets that trap the water, sugar, and other flavoring agents that you have added to your dessert. When the gelatin is cooled, the proteins remain tangled. The end result is a wiggly-jiggly solid to enjoy.

Now, back to our original question. What happens when you add fruit to your gelatin? Some fruits, like strawberries, oranges, and apples, are a tasty addition; the gelatin solidifies around the chunks of fruit. But if you add fruits like pineapple, guava, mango, or kiwi, you end up with a runny mess that never solidifies. Figure 2 below shows some of these fruits in an orange gelatin dessert. It turns out that this second group of fruits all contain proteases (pronounced PROH-tee-ay-siz), like papain (pronounced puh-PIE-uhn) and bromelain (pronounced BROH-muh-lin). Proteases are enzymes, which are a special group of proteins that help make certain chemical reactions happen. Proteases specifically act like a pair of scissors, helping reactions take place that cut other proteins up. Could it be that the papain and bromelain in these fruits are cutting the gelatin proteins into such small pieces that they are no longer able to tangle together and create a solid structure? This is exactly the question you will tackle in this science fair project.

Figure 2.
Some fruits, like strawberries (on the left), allow gelatin to solidify around them, while others, like pineapple (on the right), do not allow the gelatin to solidify. Pineapples contain special enzymes called proteases. In this science project you will investigate whether the proteases are responsible for the gelatin not solidifying.

In this food science project, you will see for yourself whether one of these protease-containing fruits interferes with gelatin's ability to solidify and, if it does, you will test whether it is the protease that interferes with solidifying by inactivating the protease (which means to make it stop working) in the fruit and then adding the fruit to the gelatin. You might be wondering how you will possibly inactivate the protease yourself. Remember that proteases, like papain and bromelain, are also proteins themselves. Most proteins can be inactivated using a variety of methods. One such method is called denaturation. Denaturation changes the structure, or shape, of the protein, without changing what it is made up of. Exposure to heat is one method of denaturing proteins. A good example of this process is cooking an egg. When the egg is raw, the egg white— which has lots of proteins, called albumins— is transparent and liquid; but after cooking, it becomes opaque and solid. In the case of eggs (and most proteases), denaturation of the protein causes an irreversible change. The heat will permanently inactivate the fruit's protease. Does this allow the gelatin to solidify or not? Ready to find out? If so, get out your spoon, because with this science fair project, you will get to enjoy some of your results as dessert!

Terms and Concepts

Gelatin

Protein

Collagen

Enzyme

Protease

Papain

Protein denaturation

Baseline

Negative control

Positive control

Experimental condition

Questions

Of what is gelatin made?

How does gelatin solidify?

What do proteases do? Why are they considered enzymes?

Which fruits contain papain, bromelain, or another protease?

Papain and bromelain are often called natural "meat tenderizers." Why is this? What does it mean? How do they tenderize meat?

What are commercial meat tenderizers, like the ones in the spice aisle of the grocery store, made out of?

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Materials and Equipment

Clear plastic cups, each at least 12 oz. in size (18); Note: If you would like to test additional fruits, add 6 cups for each type of fruit.

Permanent marker

Fresh fruit (3 cups of each fruit you would like to test); choose at least one fruit with protease and one fruit without protease from the lists below. Note: Use only fresh fruits, unless you are trying one of the Variations at the end of the experiment.

Fruits without protease, such as apples, blueberries, oranges, raspberries, and strawberries

Knife

Cutting board

Meat tenderizer (3 teaspoons); available in the spice aisle of grocery stores

Pot, large enough to hold 18 cups of hot liquid

Optional: Fruit/vegetable steamer; see the Experimental Procedure for more details.

Teaspoon measuring spoon

Dry measuring cup, 1-cup volume

Small bowl or cup

Spoons for stirring (6)

Stovetop

Oven mitt

Gelatin mix, such as JELL-O® (enough to make 18 cups of gelatin); any flavor is fine. See package details to determine how many packages you will need to yield 18 cups. Note: Each additional fruit you try will require another 6 cups of gelatin.

Experimental Procedure

Project Overview

For this science fair project, you will have several controls and test conditions. For more information about controls, read the following. You might also want to read the Science Buddies guide to
Experimental Procedures. Each condition will be made in triplicate, meaning three cups of gelatin for every condition, to ensure that your results are repeatable.

A baseline sample is one to which you can compare all other test cases. In this experiment, the baseline controls are the gelatin samples with no fruit added to them. You will compare the consistency of all other gelatin samples to these baseline controls.

A negative control is a case where you expect to see no change from the baseline. In this science fair project, the negative controls are the gelatin samples with fruit that do not have protease. Both the raw and the cooked samples will be negative controls. What do you expect the consistency of these samples to be compared to the baseline samples?

A positive control is a case where you expect to see a known change from the baseline. In this science fair project, the positive controls are the gelatin samples to which you add meat tenderizer. Meat tenderizers contain proteases, like papain and bromelain; the exact protease depends on the brand. What do you expect the consistency of these samples to be, compared to the baseline samples?

The test or experimental conditions are the ones that you have a hypothesis about that needs testing. In this science fair project, the experimental conditions are the gelatin samples containing either raw or cooked fruit that do have protease.

Testing the Different Gelatin Conditions

Label the 18 cups according to their contents. Each gelatin condition should have a #1, a #2, and a #3 cup.

Plain gelatin: Total of three cups

Raw [name of fruit]: Total of six cups—three for the fruit with protease and three for the fruit without protease

Cooked [name of fruit]: Total of six cups—three for the fruit with protease and three for the fruit without protease

Protease: Total of three cups

Cut up each of the fruits. Ask for an adult's help with this step. Be sure to wash your cutting board and knife after you cut each fruit.

Cook 1 ½ cups of each type of fruit. Fruit should be either steamed or boiled for 5 minutes. Leave the other 1 ½ cups raw.

Add approximately ½ cup of fruit (cooked or raw) to each of the plastic cups that are labeled as containing fruit.

Put 3 teaspoons (tsp.) of meat tenderizer in a small bowl. Add 3 tsp of water and mix until the tenderizer has dissolved. Put 1 tsp. of the meat tenderizer solution in each plastic cup labeled Protease.

Make the gelatin according to the package instructions. Prepare enough to make 18 cups of gelatin. Add 1 cup of gelatin liquid to each of the plastic cups. Using a spoon, thoroughly stir the contents of each cup. Make sure to use a different spoon for each condition. You will need a total of six spoons.

Refrigerate all of the cups, noting the time at which you put them inside the refrigerator in your lab notebook.

Check the consistency of the gelatin in each cup at regular intervals (once or twice an hour) while the gelatin is solidifying. Be consistent with when you check.

Examine the gelatin carefully and record your observations in your lab notebook. You may want to create a data table (listing each cup you are testing and its conditions) in your lab notebook to record your results and observations over time.

In which conditions does the gelatin set? In which conditions does the gelatin remain a liquid? Are there any in-between cases? Do your results make sense to you, and do they support your hypothesis?

What do your results tell you about how the proteases affect how the gelatin solidifies, and how heat affects the proteases?

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Variations

Does freezing fruit denature protein-digesting enzymes? Design an experiment to compare the enzyme activity of fresh and frozen pineapple.

Do other methods of fruit processing denature enzymes? How about drying or canning? Design an experiment to find out.

Design an experiment to test which type of fruit has the most enzyme activity per unit weight.

Does the meat tenderizer's effect on the gelatin depend on how much tenderizer is added? Can meat tenderizer be denatured to neutralize the effect?

Do research on alternative methods of protein denaturation, or other methods for inactivating enzymes. Choose one or more of these methods and see if it works on protein-digesting enzymes in fruit. (Do not eat the results of this experiment!)

Enzyme activity can be affected by pH. You could try mixing the protease-containing fruits and liquid gelatin with acids or bases to change the pH of the mixture and then see how this affects the protease activity. Be sure to follow all safety precautions when using potentially dangerous chemicals and ask for adult assistance if needed. There are a lot of common acids and bases you may be able to use that are around your home, and you can purchase pH sticks to measure the pH of your mixtures. To find out more about how you can do this variation, see: Acids, Bases, & the pH Scale. (Be sure to include controls that contain only the liquid gelatin and the acid or base to make sure the pH alone is not affecting how well the gelatin solidifies.)

At some health foods stores, you can find a product called "vegan gelatin." You could repeat the experiment in this science project but this time compare vegan gelatin to regular gelatin. (You could skip using the cooked fruit.) Do the protease-containing fruits interfere with the vegan gelatin's ability to solidify in the same way that they do with regular gelatin, or do you get different results using vegan gelatin? You may want to do some background research on what vegan gelatin is made of to come up with your hypothesis. Variation credits: This idea was submitted by Riya C., New York.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Related Links

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